1. Field of the Invention
The present invention relates to a disk driving apparatus for driving a disk into rotation.
2. Description of the Prior Art
In the conventional disk driving apparatus for a disk having a circular-plate shape such as a compact disc (CD), a compact disc read-only memory (CD-ROM) or an optical magnetic disk (MO), in order to read out and write information from and to the circular-plate disk which is referred to hereafter simply as an optical disk, it is necessary to maintain the positional relationship between the optical disk and an optical pickup for implementing the operation to read out and write the information with a high degree of precision. If a positional shift occurs between the optical disk and the optical pickup, it becomes difficult to read out and write information with a high degree of accuracy. This causes problems in a playback operation, for example, the sound quality of a reproduced audio signal and the picture quality of a reproduced video signal deteriorate.
In particular, since the optical disk described above is generally made of compound resin, there are observed relatively big variations in the diameter of the hole provided at the center of the optical disk for facilitating the mounting of the optical disk on a turntable. Thus, a measure for countering these variations and, hence, for suppressing a positional shift, is absolutely required.
An example of such a countermeasure is a centering mechanism of an optical disk which is provided so as to always mount the optical disk at a correct position on the turntable.
An example of an optical-disk playback apparatus with a centering mechanism is shown in FIGS. 5 and 6. FIG. 5 is a diagram roughly showing a side view of the principle components of a conventional optical-disk playback apparatus. FIG. 6 is a diagram roughly showing a plane view of the conventional optical-disk playback apparatus shown in FIG. 5.
As shown in FIGS. 5 and 6, the optical-disk apparatus 1 for playing back information from a CD comprises a spindle motor 2, a turntable 3 and a clamp 4.
The spindle motor 2 comprises a rotor 2a and a stator board 2b which is installed on a chassis 2c by screws 2d.
The chassis 2c is mounted on a main-body frame of the optical-disk apparatus 1 through insulators 2e each made of rubber so as to prevent vibration from being propagated from the spindle motor 2 to the optical disk. It should be noted that the main-body frame itself is not shown in the figure.
The turntable 3 is fixed on the upper end of a rotational shaft 2f of the spindle motor 2 concentrically. An optical disk 5 can be mounted on and removed from the turntable 3 freely.
Formed at the center of the turntable 3 is a circular protrusion 3a used as a horizontal-direction reference for mounting the optical disk 5 with a circular-plate shape on the turntable 3. The optical disk 5 has a circular hole 5a bored through the center thereof. The optical disk 5 is mounted on the turntable 3 with the protrusion 3a inserted through the hole 5a.
The clamp 4 has a circular shape in the horizontal direction. At the center of the lower surface of the clamp 4, a circular dent is formed for accommodating the protrusion 3a of the turntable 3. The clamp 4 is placed above the turntable 3 and the optical disk 5.
At the inner part of the dent of the clamp 4 described above, a magnet 4a is mounted. The magnet 4a is attracted by the upper surface of the protrusion 3a of the turntable 3 so that the lower surface of the circumference of the clamp 4 is pressed against a portion of the optical disk 5 facing the circumference. In this way, the optical disk 5 is sandwiched firmly between the clamp 4 and the turntable 3, being fixed on the turntable 3.
In addition, an optical pickup 6 for reading and writing information from and to the optical disk 5 is mounted slidably in the radial direction of the optical disk 5 along guide axes 7a installed on the chassis 2c as shown in FIG. 6. The optical pickup is driven by a sled motor 7b through a sled gear 7c into sliding motion in the radial direction.
A centering mechanism for the optical disk 5 is provided on the protrusion 3a of the turntable 3. The centering mechanism has a configuration shown in FIGS. 7 and 8. FIG. 7 is a diagram showing, in an enlarged form, a cross section of the protrusion 3a at the center of the turntable 3 employed in the optical-disk playback apparatus shown in FIG. 5. FIG. 8 is a diagram showing partially, in an enlarged form, the centering mechanism of the optical disk 5 on the turntable 3.
As shown in FIGS. 7 and 8, the protrusion 3a at the center of the turntable 3 includes a base 8a, a ring 8b, a coil spring 8c and a chalking yoke 8d.
The base 8a is created at the center of the turntable 3 to enclose a bearing pressed against the rotational shaft 2f of the spindle motor 2. The base 8a is engaged with and fixed to the bearing. The coil spring 8c is engaged with the base 8a and the ring 8b is engaged with the coil spring 8c from a position above the coil spring 8c.
On the upper edge of the base 8a, the chalking yoke 8d having a ring-like shape is provided for attracting the magnet 4a of the clamp 4. In the embodiment shown in the figures, the chalking yoke 8d is created to form a single body with the base 8a.
The chalking yoke 8d has an inner diameter smaller than the outer diameter of the ring 8b and a circumferential edge bent downward along the entire circumference thereof.
The inner diameter of the ring 8b is made slightly larger than the outer diameter of the base 8a by such a difference that the ring 8b can be moved up and down relatively to the base 8a with the inner circumferential surface of the ring 8b sliding over the outer circumferential surface of the base 8a with no backlash with respect to the base 8a.
The outer surface of the ring 8b is inclined downward on the outer side in the radial direction thereof. That is to say, the ring 8b has a conical shape which has a minimum outer diameter and a maximum outer diameter on the top and bottom surfaces thereof respectively.
In addition, the ring 8b is created so that the outer diameter at a middle position between the top and bottom surfaces thereof is about equal to the diameter of the circular hole 5a bored through the center of the optical disk 5 to be mounted.
On the lower surface of the ring 8b, a continuous groove having a ring shape is created in the circumferential direction. The upper end of the coil spring 8c is plugged into the groove.
Thus, by bringing the upper end of the coil spring 8c into contact with the inner part of the groove, the ring 8b is pressed by the coil spring 8c in the upward direction.
Here, the upper limit of the position of the ring 8b is determined when the upper surface of the ring 8b comes in contact with the lower edge of the bent circumference of the chalking yoke 8d. When no optical disk 5 is mounted, the ring 8b is held at the upper limit of the position by the tension of the coil spring 8c.
When an optical disk 5 is mounted on the turntable 3 having a centering mechanism with such a configuration, first of all, the lower edge of the hole 5a of the optical disk 5 is brought into contact with a circumferential portion of the ring 8b in close proximity to the middle between the top and bottom circumferential surfaces of the ring 8b as shown in FIG. 8. The ring 8b is pulled downward by the weight of the optical disk 5, resisting the tension of the coil spring 8c.
Later on, as the clamp 4 is engaged with the protrusion 3a of the turntable 3 as described above, the magnet 4a of the clamp 4 is attracted by the chalking yoke 8d. In this way, the optical disk 5 is further pulled by a pressure generated by the chalking yoke 8d in the downward direction along with the ring 8b. As a result, the lower surface of the optical disk 5 is brought into contact with the upper surface of the turntable 3, being firmly held on the turntable 3.
In this way, by providing the ring 8b with an outer surface having a conical shape that can be slided up and down under the pressure of the tension generated by the coil spring 8c, any variations in diameter of the hole 5a bored at the center of the optical disk 5 are absorbed by the up-and-down movement of the spring 8b when the optical disk 5 is mounted on the turntable 3, making it possible to prevent the optical disk 5 from being mounted on the turntable 3 in an eccentric state with a high degree of reliability.
In the turntable 3 having a centering mechanism with a configuration described above, however, a play margin between the ring 8b and the base 8a is required so as to allow the ring 8b to be slided up and down with respect to the base 8a. Unfortunately, a shift is inadvertently result may between the centers of the ring 8b and the base 8a, that is, between the centers of the optical disk 5 and the turntable 3. The shift between the centers becomes a cause of deterioration of the accuracy and the characteristics of the operations to read out and write information by the optical pickup 6.
In addition, since a space for allowing the ring 8b to be slided up and down is required, as a whole, the height of the turntable 3 increases, giving rise to a problem caused by the fact that the increase in height of the turntable 3 is disadvantageous to the effort to make the entire optical-disk apparatus compact and thin.
Moreover, since the optical disk 5 mounted on the turntable 3 is always pressed in the upward direction by the tension of the coil spring 8c through the ring 8b, the chalking force of the magnet 4a is weakened. Thus, in order to avoid a slip between the optical disk 5 and the turntable 3, it is necessary to employ a strong magnet 4a. As a result, the magnet 4a itself becomes large in size and expensive, opposing the attempt to make the entire optical-disk apparatus small in size and have a low cost.
Furthermore, since the ring 8b and the coil spring 8c are required in the centering mechanism of the optical disk 5, the number of components increases, raising the component and assembly costs as well as lengthening the time it takes to assemble the components. As a result, such a centering mechanism becomes the cause of a decrease in production efficiency.
Further, in recent years, a multilayered optical disk for high-density recording applications such as disks stuck to each other like the ones shown in FIG. 9 is under development. FIG. 9 is a diagram showing cross sections of the main components composing a multi-disk set. As an example, a DVD (a digital versatile disk) is explained as follows.
In the example shown in FIG. 9, two optical-disk plates 9a and 9b are stuck by adhesive to each other to form a multi-disk set 9. As shown in the figure, the optical-disk plates 9a and 9b which overlap each other have holes 9a1 and 9b1 respectively bored through the centers thereof. The holes 9a1 and 9b1 have a typical diameter of 15 mm.
According to the specifications of the DVD, as a stand-alone optical-disk plate, the allowable range of shifts in inner diameter of the optical-disk plates 9a and 9b is +0.15 mm to -0 mm. After they are stuck to each other, however, the inner diameter of the hole must be at least 15 mm. Thus, in a worst case, the multi-disk set 9 has a maximum shift allowable by the specifications shown in FIG. 10. FIG. 10 is a diagram showing cross sections of optical-disk plates of a multi-disk set which are shifted from each other by a maximum allowable distance. As shown in the figure, the optical-disk plates 9a and 9b with holes 9a1 and 9b1 respectively both have an inner diameter of 15.15 mm and are stuck to each other in a state with the optical-disk plates 9a and 9b shifted from each other in mutually opposing directions.
Thus, in an actually produced multi-disk set 9, in general, the holes 9a1 and 9b1 of the optical-disk plates 9a and 9b are shifted slightly from each other as shown in FIG. 11. FIG. 11 is a diagram showing cross sections of optical-disk plates of a multi-disk set which are shifted from each other by a typical distance. When the centering mechanism described above is applied to such a multi-disk set, the holes 9a1 and 9b1 of the optical-disk plates 9a and 9b which should each serve as a standard of centering are shifted to each other, giving rise to a problem that centering can not be accomplished with a high degree of accuracy.